Tracking systems serve to determine and track positions of devices by detecting (e.g. by means of a camera) a position of markers attached to the device. Medical tracking systems serve to determine a position of an instrument or body part with the aid of markers and/or a reference star attached to the instrument/body part (e.g., the reference star can include the markers). The markers can be passive or active markers. Passive markers reflect light, in particular infrared light (but also, for example, visible or UV light) radiated onto them, while active markers emit light themselves. The position of the markers, in particular the reference star and therefore the instrument or body part attached to the reference star, can be determined with the aid of cameras that detect light emitted by the markers.
The markers typically are spherical markers, but also can exhibit a different structure, such as cubiform, for example. A surface of passive markers generally is formed similar to cats' eyes, with a multitude of small reflecting faces that in turn can be spherical or hemispherical. Thus, a multitude of recesses can be situated on the markers, in which liquid, such as, for example, rinsing liquid, blood or blood smears, can accumulate. These liquids or soils can alter the emission of light from the markers and, as a result, the navigation system may incorrectly calculate the position of the markers. In the case of spherical markers, for example, a center of the marker sphere can be calculated incorrectly if the emitted light (active or reflected light) is changed by marks or drops of liquid situated on the markers. In particular, additional reflections may occur on the surface of the liquid, which impair the emission or light, resulting in calculation errors.
In practical medical applications, the multitude of recesses in markers are problematic, as it is difficult for the practitioner (in particular a physician or nurse) to recognize whether the reflections are due to liquid or marks on the marker. The practitioner thus does not know whether it is necessary to clean the markers. In particular, the recesses spread the liquid out on the surface of the marker and, thus, make it more difficult to determine whether the marker should be cleaned.